NCERT Solutions for Class 8 Science<\/a> Chapter 15 Some Natural Phenomena Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.<\/p>\nSome Natural Phenomena NCERT Solutions for Class 8 Science Chapter 15<\/h2>\nClass 8 Science Chapter 15 Some Natural Phenomena Textbook Exercise Questions and Answers<\/h3>\n
Page-197<\/span><\/p>\nSelect the correct option in Questions 1 and 2.<\/span><\/p>\nQuestion 1.
\nWhich of the following cannot be charged easily by friction?
\na. A plastic scale
\nb. A copper rod
\nc. An inflated balloon
\nd. A woollen cloth
\nAnswer:
\nb. A copper rod<\/p>\n
Question 2.
\nWhen a glass rod is rubbed with a piece of silk cloth the rod
\na. and the cloth both acquire positive charge
\nb. becomes positively charged while the cloth has a negative charge
\nc. and the cloth both acquire negative charge
\nd. becomes negatively charged while the cloth has a positive charge
\nAnswer:
\nb. becomes positively charged while the cloth has a negative charge<\/p>\n
Question 3.
\nWrite T against true and F against false in the following statements:
\na. Like charges attract each other. (T\/F)
\nb. A charged glass rod attracts a charged plastic straw. (T\/F)
\nc. Lightning conductor cannot protect a building from lightning. (T\/F)
\nd. Earthquakes can be predicted in advance. (T\/F)
\nAnswer:
\na. F,
\nb. T,
\nc. F,
\nd. F<\/p>\n
Question 4.
\nSometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
\nAnswer:
\nWhen we take off a sweater, it rubs against our shirt. The rubbing together of sweater and shirt produces opposite electric charges on them. The discharge of these electric charges produces tiny sparks of light as well as crackling sound.<\/p>\n
<\/p>\n
Question 5.
\nExplain why a charged body loses its charge if we touch it with our hand.
\nAnswer:
\nHuman body is a conductor of electricity. So, when we touch a charged object, our body conducts its charges to the earth. That is why a charged body loses its charge, if we touch it with our hand. This phenomenon is known as electric discharge.<\/p>\n
Question 6.
\nName the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?
\nAnswer:
\nThe destructive energy of an earthquake is measured by the Richter scale. The reading of magnitude 3 on the Richter scale would be recorded by a seismograph. If the Richter scale gives a reading of magnitude 3, the earthquake is not likely to cause much damage. Generally, earthquakes having magnitudes higher than 7 are destructive in nature.<\/p>\n
Question 7.
\nSuggest three measures to protect ourselves from lightning.
\nAnswer:
\nProtective measures against lightning are as follows:<\/p>\n
\n- Stay in a completely closed place. If you are moving in a car, remain there till the lightning is over. Close the windows of the car immediately.<\/li>\n
- Do not touch any electrical wires, telephone cables, metal pipes, etc.<\/li>\n
- Do not take a bath in running water. This may cause an electric shock.<\/li>\n<\/ul>\n
Question 8.
\nExplain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
\nAnswer:
\nWe know that like charges repel each other. Since both the balloons carry similar charges, they are repelled by each other. But a charged balloon attracts an uncharged balloon because a charged object has a tendency to attract an uncharged object.<\/p>\n
Question 9.
\nDescribe with the help of a diagram an instrument which can be used to detect a charged body.
\nAnswer:
\nAn electroscope can be used to detect whether a body is charged or not. The figure given here shows a simple electroscope.
\n
\nIt consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc. The leaves of aluminium foil are placed inside a closed jar to isolate the leaves from air.<\/p>\n
When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because some of the charges of the body are transferred to the strips through the metal rod. The nature of charges on both the leaves is similar. Hence, both the leaves of the aluminium foil move away from each other. If, however, the body is not charged, the two leaves remain as they are and will not repel each other.<\/p>\n
<\/p>\n
Question 10.
\nList three states in India where earthquakes are more likely to strike.
\nAnswer:
\nThe three states in India where earthquakes are more likely to strike are Jammu and Kashmir, Gujarat and Assam.<\/p>\n
Question 11.
\nSuppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
\nAnswer:
\nIf we are out in the open and an earthquake strikes, we can take the following precautions:<\/p>\n
\n- We should try to move away from buildings and other structures.<\/li>\n
- If we are in a vehicle, we should ask the driver to drive slowly to a safe spot.<\/li>\n
- We should keep away from bridges.<\/li>\n<\/ul>\n
Question 12.
\nThe weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day, would you carry an umbrella? Explain.
\nAnswer:
\nIf a thunderstorm is being predicted, we should not carry an umbrella. The metallic handle of an umbrella can be a potential target for lightning, and therefore, it is unsafe to carry an umbrella.<\/p>\n
NCERT Extended Learning Activities and Projects<\/span><\/p>\nQuestion 1.
\nOpen a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity.
\nHint:
\nEven if the water molecules as a whole are neutral, one of their ends is negatively charged while the other end is positively charged. Once the water molecule comes in contact with a charged object, it will get attracted towards it.<\/p>\n
Question 2.
\nMake your own charge detector. Take a paper strip roughly 10 cm \u00d7 3 cm. Give it a shape as shown in the figure. Balance it on a needle. Bring a charged body near it. Observe what happens. Write a brief report, explaining its working.
\n
\nHint:
\nWhen a charged body is brought near it, it starts moving. When the charged body is rotated in a circular way, it also shows a circular motion.<\/p>\n
<\/p>\n
Question 3.
\nThis activity should be performed at night. Go to a room where there is a fluorescent tube light. Charge a balloon. Switch off the tube light so that the room is completely dark. Bring the charged balloon near the tube light. You should see a faint glow.
\nMove the balloon along the length of the tube and observe how the glow changes.
\nCaution: Do not touch the metal parts of the tube or the wires connecting the tube with the mains.
\nHint:
\nWhen a charged balloon touches the tubelight, electrons pass from the balloon to the tubelight causing the bulb to emit small sparks of light. Under normal circumstances, the tubelight receives electrons from the electric power lines through a wire at the end of the tube.<\/p>\n
Question 4.
\nFind out if there is an organisation in your area which provides relief to those suffering from natural disaster. Enquire about the type of help they render to the victims of earthquakes. Prepare a brief report on the problems of the earthquake victims.
\nHint:
\nStudents should find out about the organisations in their respective areas which provide relief to those suffering from a natural disaster. An earthquake
\nmay cause damage and destruction to life and property. Roads and bridges collapse. Earthquake victims have to face deficiencies of food, water, security, medicine, fuel and so on, apart from the loss of lives of their loved ones.<\/p>\n
Activity 1<\/span><\/p>\nObjective: To collect the different objects and charge them by rubbing.
\nMaterials Required: Refill, balloon, eraser, steel spoon, coin, ball pen, polythene, woollen cloth, dry hair.
\nProcedure and Observations:<\/p>\n
\n- Collect the different articles listed above and rub them with the objects as shown in the table given below.<\/li>\n
- Try to attract small bits of paper with them and observe what happens in each case.<\/li>\n
- Use insulating gloves while charging the metal spoon and coin.<\/li>\n<\/ul>\n
\n\n\n\n Object rubbed<\/p>\n<\/td>\n | Material used for rubbing<\/td>\n | Attract\/ Does not attract bits of paper<\/td>\n | \n A Charged\/ Un\u00ad charged<\/p>\n<\/td>\n<\/tr>\n |
\n1. Refill<\/td>\n | Polythene, woollen cloth<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n |
\n2. Bal\u00adloon<\/td>\n | Polythene, woollen cloth, dry hair<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n |
\n3. Eraser<\/td>\n | Wool<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n |
\n4. Steel spoon<\/td>\n | Polythene, woollen cloth<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n |
\n5. Coin<\/td>\n | Woollen cloth<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n |
\n6. Ball Pen<\/td>\n | Hair<\/td>\n | Attracts<\/td>\n | Charged<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Conclusion: Objects can be charged by rubbing.<\/p>\n \n- Positive and Negative Charges: There are two types of charges namely positive charge and negative charge. Like charges repel each other while unlike charges attract each other.<\/li>\n
- It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. The other kind of charge is said to be negative.<\/li>\n<\/ul>\n
<\/p>\n Activity 2<\/span><\/p>\nObjective: To show that like charges repel each other, whereas unlike charges attract each other. \nMaterials Required: Two glass rods, two ebonite rods, a piece of silk and a piece of fur. \nProcedure:<\/p>\n \n- Rub a glass rod with a piece of siik and hang it with a thread (Fig. a).<\/li>\n
- Rub another glass rod with a piece of silk and bring it near the suspended charged glass rod. See what happens.<\/li>\n
- Rub an ebonite rod with a piece of fur and hang it with a thread (Fig. b).<\/li>\n
- Rub another ebonite rod with a piece of fur and bring it near the suspended charged ebonite rod and observe.<\/li>\n<\/ul>\n
\n \n \nNow perform similar activity by bringing a charged ebonite rod near a suspended charged glass rod (Fig. c).<\/p>\n
Observation: Charged glass rod moves away s from another charged glass rod and charged ebonite rod moves away from another charged ebonite rod. But. a charged glass rod gets attracted towards a charged ebonite rod. \nConclusion: When the charges repel each other, they are like charges, and when the charges attract each other, they are unlike charges.<\/p>\n The electrical charges produced by rubbing are called static charges. When charges move, they constitute an electric current.<\/p>\n Transfer of Charge: Electric charge can be transferred from a charged object to another charged or uncharged object through a metal conductor.<\/p>\n Electroscope: It is a device that can be used to test whether an object is carrying any charge or not. An electroscope consists of two closely placed metallic (aluminium) foils or strips. When an electroscope is charged, its aluminium strips are diverged. If we touch the metal top of a charged electroscope with our finger, it gets discharged and its aluminum strips collapse. This is because the electric charge present on the charged electroscope flows to the earth through our hand and body. The electroscope loses all the electric charge, becomes uncharged and its diverged aluminium strips collapse. \n<\/p>\n Discharging: When a charged object loses its charge, it is said to be discharged. Earthing: When a charged object transfers its charge to the earth, it is called earthing. Generally, every building is provided with an earthing to protect it from electrical shocks due to leakage of electric current.<\/p>\n The Story of Lightning: \ni. During the formation of a thunderstorm, the air currents move upward and the water droplets move downward. These vigorous movements result in separation of charges in the clouds.<\/p>\n ii. From these generated charges, the positive charges accumulate near the upper edges of the clouds and the negative charges accumulate near the lower edges of the clouds. Scientists are yet to understand the exact reason for this. There is an accumulation of positive charges near the ground also.<\/p>\n iii. When accumulation of charges becomes very large, the air which is a bad conductor of electricity under normal environment, is no longer able to resist their flow. As a result, the electric charges meet and produce streaks of bright light and sound across the sky. This process is called an electric discharge. It can occur between two or more clouds or between the clouds and the earth. \n<\/p>\n Dangers of Lightning: Lightning can damage houses and trees. It can also kill people and cattle.<\/p>\n Lightning Safety: One should not stay at an open place during lightning and thunderstorm. Hence as soon as we hear any alert about lightning or thunderstorm, we should rush to a safe place such as a house or a building. If somebody is there in a car or bus, they should stay inside and keep all the doors and windows closed. One should remain inside the safe place until the storm lasts.<\/p>\n Do\u2019s and Don\u2019ts during a Thunderstorm: \nOutside the house-<\/p>\n \n- Open vehicles, like motorbikes, open cars, etc., are not safe.<\/li>\n
- Open fields, tall trees, parks and elevated places are also not safe.<\/li>\n
- Do not carry an umbrella during a thunderstorm.<\/li>\n
- In a forest, take shelter under shorter trees.<\/li>\n
- In an open field, stay away from trees, poles and other metal objects.<\/li>\n
- Squat low on the ground, place your hands on your knees with your head between the hands. This will make you the smallest target to be hit.<\/li>\n<\/ul>\n
<\/p>\n Inside the house-<\/p>\n \n- Avoid contact with electrical switches and appliances.<\/li>\n
- It is safer to use cell phones or cordless phones instead of a wired phone.<\/li>\n
- Bathing should be avoided during thunderstorms to avoid contact with running water.<\/li>\n
- Electrical appliances should be unplugged.<\/li>\n<\/ul>\n
Lightning Conductor: This is a simple device which protects a building from the effect of lightning. It is composed of a vertical metallic rod which usually has a trident at the top and is taller than the building. The base of the metallic rod is attached to a thick metallic plate which goes very deep inside the earth. This metallic wire provides a passage for earthing. When lightning strikes, the electric charge goes to the lightning conductor which transfers it safely to the earth. Thus, a lightning conductor helps in preventing any damage to the building. \n<\/p>\n Earthquake: Sudden shaking of the earth which lasts for a very short duration is called earthquake. Earthquake can cause immense damage to buildings, dams, etc. They may also cause floods, landslides, tsunamis and loss of life.<\/p>\n A major earthquake occurred in India on 8 October 2005, in Uri and Tangdhar towns of North Kashmir. Before that, a major earthquake occurred on 26 January 2001, in Bhuj district of Gujarat. \n<\/p>\n Cause of an Earthquake: It is caused by a disturbance deep inside the earth’s crust. There are three layers of the earth-the outer crust, the middle mantle and the inner core. The outermost layer of the earth is not in one piece. It is fragmented and each fragment is called a plate or earth plate. These plates are in continuous motion. When these plates collide with one another, one plate goes under another plate which causes disturbance in the earth\u2019s crust. Due to this disturbance, tremors take place in the form of earthquakes. \n<\/p>\n Seismic or Fault Zones: The earthquakes occur due to collision of plates. Therefore, the boundaries of plates are the weak zones where an earthquake is likely to occur. These week zones are called \u2018Seismic zones\u2019.<\/p>\n The places of seismic zones in India include Kashmir, Western and Central Himalayas, the whole North East, Great Rann of Kutch in the district of Gujarat, and some areas of South India. \n<\/p>\n Richter Scale: A scale which is used to determine the magnitude or strength of an earthquake is called the Richter scale. Destructive earthquakes have magnitude of more than 7 on a Richter scale.<\/p>\n Seismic Waves: The earthquakes produce tremors or waves on the surface of the earth, which are called seismic waves. These waves travel in all the directions on the earth\u2019s surface.<\/p>\n Seismograph: A seismograph is an instrument which can record the seismic waves. It contains a metal rod or a pendulum which can vibrate as the earthquake occurs. The metal rod is attached to a pen which records the waves on the paper. Scientists study these waves and then construct a map of the earthquake. This also helps them in determining the power of the earthquake. \n<\/p>\n Protection against Earthquake: The buildings located in the seismic zones should be constructed in a way that they can handle major tremors. The following measures should be taken by the people to make the houses or buildings quake-safe:<\/p>\n \n- People should always consult architects and engineers before constructing a new building.<\/li>\n
- The roofs of the building should be kept light.<\/li>\n
- Timber or mud should be used in the construction of buildings rather than heavy materials.<\/li>\n
- The shelves and cupboards should be fixed to the walls so that they do not fall off easily.<\/li>\n
- Things such as photo frames, geysers, clocks and other wall hangings should be carefully placed so that they do not harm anybody if they fall off.<\/li>\n
- Buildings should have firefighting equipment as earthquakes may cause fires.<\/li>\n
- During the earthquake, one should take the following steps to protect oneself:<\/li>\n<\/ul>\n
Inside the house:<\/p>\n |